1. Field of the Invention
The present invention relates to a highly functional electrophotographic photosensitive material, and more particularly to an electrophotographic photosensitive material which has photosensitivity suitable for use with a semiconductor laser and is easy to control its properties.
2. Description of the Prior Art
Recently, non-impact printer technique attained a great development and consequently photoprinters of electrophotography system, which enable to obtain a high definition and a high speed by using a laser beam or an LED as a light source, are now widely spreading in the market. Therefore, intensive research and development are made to try to obtain photosensitive materials which satisfy requirements of such photoprinters.
Particularly when a laser beam is used as a light source, there is a need to obtain a photosensitive material having a photosensitivity up to a near infrared region. It is because mostly a semiconductor laser apparatus is used owing to the merits of compactness, cheapness and simplicity, but their oscillation wavelength is now limited to a relatively longer wavelength range of a near infrared region. Accordingly, it is not appropriate to use conventional photosensitive materials, which have been used for electrophotographic copying machines and have a sensitivity in a visible region, for a semiconductor laser apparatus.
It is known that some organic materials satisfy the need as above mentioned. They are, for example, indoline dyes, polyazo dyes, phthalocyanine dyes, naphthoquinone dyes, etc. At present, however, the indoline dyes enable to obtain a longer wavelength range but are lacking in a utilizable stability, that is, repeating property; the polyazo dyes are difficult to obtain a longer wavelength range and also have disadvantages in its production; and naphthoquinone dyes have a problem in its sensitivity.
On the other hand, the phthalocyanine dyes have spectrum sensitivity peaks in the long wavelength region of not less than 600 nm, have high sensitivities and further change their spectrum sensitivities in accordance with the kinds of their center metals and their crystal forms. They are therefore considered as suitable for dyes for a semiconductor laser apparatus and so research and development of them are intensively conducted.
It has been attempted in these days to use titanyl phthalocyanine having relatively high sensitive electrophotographic properties as described in Japanese Patent Laid-open Nos. 49,544/84, 23,928/86, 109,056/86 and 275,272/87. According to these literatures, it is understood that their properties are different owing to their crystal forms and that in order to produce these various crystal forms, special purification and special solvent treatment are required. Further, the solvent to be used for such treatment is one that is different from the solvent to be used in forming the dispersion coating film. It is because the various crystals to be formed have tendency to easily grow in the solvent for the crystal growing treatment and so, if such solvent is used also as the solvent for the coating, it is difficult to control the crystal form and the particle size. This causes low stability of the coating material and eventually the electrostatic properties are significantly damaged. Therefore, ordinarily in the treatment of forming the coating material, a chlorine series solvent such as chloroform, which does not substantially promote the crystal growth, is used. This solvent however does not always show good dispersion property for the titanyl phthalocyanine and so causes some problems in the dispersion stability of the coating material.
As for structures of such photosensitive materials, there are a multilayer structure having a function separation type photosensitive material which includes, as separate layers, a material generating electric charge carriers (hereinafter called a charge generation material) and a material which receives the generated electric charge carriers and transports them (hereinafter called a charge transport material); and a single layer structure having a single layer type photosensitive material which executes generation of electric charge carriers and transportation of electric charge by means of the same material. The multilayer structures are adopted more than the single layer structures because the formers have the larger range of selection of materials and have higher sensitivity than the latter.
As for the charge transport material, particularly a hole transport material, there have been various proposals of photosensitive materials utilizing hydrazone compounds, butadiene compounds, poly-2,3-epoxypropyl carbazole compounds and so on, and some of them are actually used in the industry.
The prior art photosensitive materials containing hydrazone compounds have excellent electric properties, but have problems of deterioration by optical fatigue. Further, the titanyl phthalocyanine in general has a large ionization potential, and so if it is used with a material having a small ionization potential such as hydrazone compounds, hole injection from the titanyl phthalocyanine to the hydrazone compounds easily occurs owing to the large difference of ionization potential. This cause a problem of low charging ability and thus of considerable reduction of surface voltage owing to repeated used and optical fatigue coming therefrom. In case the hydrazone compounds are used for a dispersed type photosensitive material containing the charge generation material and the charge transport material in a single layer, it is very difficult to let the charge generation material be contained in a large amount for improving sensitivity, while retaining charging ability.
Further, the prior art photosensitive materials mainly consisting of butadiene compounds have good resistance to optical fatigue, but have no good electric properties.
In addition, the hydrazone compounds and the butadiene compounds do not have tendency to form a film and so it is required to dissolve them in a solvent together with a resin or a binder. This causes dilution of density and so results in poor achievement of their functions.
Further, the prior art photosensitive materials utilizing poly-2,3-epoxypropyl carbazole compounds alone do not show good film-formation property and moreover have ionization potential larger than that of the titanyl phthalocyanine. Therefore, hole injection is difficult to occur and so mobility in negative charge is made slow to invite tendency to elevate a residual voltage.